1. Field of the Invention
The present invention relates to an electromagnetic valve.
2. Discussion of Background
FIGS. 6 and 7 are side cross-sectional views illustrating a conventional electromagnetic valve. FIG. 6 illustrates a state that a spool valve is moved to be in a state that a flow passage on a side of an input port is fully opened at a time that a coil is not applied with a current. FIG. 7 illustrates a state that the spool valve is moved to be in a state that a flow passage on a side of a drain port is fully opened at a time that a current is applied to the coil. In the figures, numerical reference 21 designates a bobbin; numerical reference 22 designates a coil wound around the bobbin 21; numerical reference 23 designates a movable iron core; numerical reference numeral 23a designates a rod inserted in and fixed to the movable iron core 23; numerical reference 24 designates a core forming a magnetic circuit; numerical reference 25 designates a case forming the magnetic circuit in a manner similar to that in the core 24 and accommodating the above-mentioned component; numerical reference 26 designates a cap regulating a movement of the movable iron core 23 in a direction opposite to that of the core 24; numerical reference 27 designates a spool valve pushed by the rod 23a; numerical reference 28 designates a housing, in which the spool valve 27 inserted in the housing slides on a bore thereof; numerical reference 28a designates an input port; numerical reference 28b designates an output port; and numerical reference 28c designates a drain port.
Numerical reference 29 designates a spring urging the movable iron core 23 and the spool valve 27 in a direction of the cap 26; numerical reference 30 designates an adjust screw adjusting a force of the spring 29; numerical reference 31 designates a feed-back chamber formed by steps of the spool valve 27 and the housing 28; numerical reference 32 designates a feed-back choke introducing an output pressure to the feed-back chamber 31; numerical reference 33a designates a spool land as a part of the spool valve 27 for sealing leakage from the input port 28a to the feed-back chamber 31 in collaboration with the housing 28; numerical reference 33b designates a spool land as a part of the spool valve 27 for sealing leakage from the feed-back chamber 31 to the core 24 in collaboration with the housing 28; numerical reference 34a designates a housing land as a part of the housing 28 for sealing leakage from the input port 28a to the feed-back chamber 31 in collaboration with the spool valve 27; and numerical reference 34b designates a housing land as a part of the housing 28 for sealing leakage from the feed-back chamber to the core 24 in collaboration with the spool valve 27.
In the next, an operation will be described. As illustrated in FIG. 6, when a current is not applied, the spool valve 27 is positioned at a point where the force of the spring 29 is balanced with a feed-back force, generated by the output pressure introduced into the spool valve feed-back chamber 31 through the feed-back choke 32, wherein a pressure corresponding to an initial set force of the spring 29 is applied to the output port 28b. When the current is applied to the coil 22, as illustrated in FIG. 7, an electromagnetic force attracting the movable iron core 23 in a direction of the core 24. Therefore, the balancing point is changed, and a rate of choking the input port 28a and the drain port 28c is changed by the spool valve 27, whereby the output pressure is decreased. Accordingly, the output pressure is in proportion to the current applied to the coil 22.
The conventional electromagnetic valve is constructed so that the output pressure is determined by balancing of the spring force, the feed-back force, an electromagnetic force, and feed-back force and the output pressure is in a relationship of 1:1. However, if lengths of seals respectively between the input port and feed-back chamber and of the feed-back chamber and the core are changed upon movement of the spool valve, the feed-back force, i.e. pressure, becomes independent of the output pressure by leakages from the input port to the feed-back chamber and from the feed-back chamber to the core, whereby the output pressure is not sufficiently increased even though a current is decreased to reduce an electromagnetic force, as illustrated in FIG. 8.
Further, when a pressure applied to the input port is increased, balancing of the pressure around the spool valve is disturbed, and a phenomenon called hydraulic lock, i.e. the spool valve is pushed to the housing, occurs to disturb balancing of the spring force, the feed-back force, and the electromagnetic force. Thus, there is a problem that the output pressure corresponding to a designated current is not obtainable.
It is an object of the present invention to solve the above-mentioned problems inherent in the conventional technique and to provide an electromagnetic valve which constantly and stably outputs an output pressure even though a supply pressure is high.
According to a first aspect of the present invention, there is provided an electromagnetic valve comprising: a bobbin, accommodated in a case and wound by a coil; a core located in a central portion of the bobbin and forming a magnetic circuit; a movable iron core, sucked in a direction of the core when a current is applied to the coil; a rod, integrally assembled with the movable iron core; a spool valve, pushed by the rod; a housing guiding the spool valve; and a feed-back chamber, formed by steps of the spool valve and of the housing, wherein a sealing portion formed by the spool valve and the housing between an input port and the feed-back chamber has a length, which is not changed by movement of the spool valve.
According to a second aspect of the present invention, there is provided the electromagnetic valve, wherein a length of a sealing portion between the feedback chamber and the core is not changed by movement of the spool valve.
According to a third aspect of the present invention, there is provided the electromagnetic valve, wherein a groove is formed in an inner periphery of the housing between the input port and the feed-back chamber.
According to a fourth aspect of the present invention, there is provided the electromagnetic valve, wherein a groove is formed in an inner periphery of the housing between the feed-back chamber and the core.